This invention relates to new and useful improvements in breakaway couplings for the bases of pole structures. Although the couplings are designed primarily for light poles normally encountered upon streets and roads, other structures such as traffic lights, street signs, display boards and other roadside structures might also employ this invention.
Conventionally, such poles are made of metal, concrete, fiber-reinforced plastic or another such strong and durable material in either a cylindrical or polygonal cross-sectional hollow form. They are normally quite tall in height and usually include an offset portion to support light fixtures. Additionally, the poles are designed to be long-lasting and highly resistant to corrosion and adverse weather conditions, including rain, snow, and normal wind shear.
When a vehicle strikes such a pole, the rapid deceleration of the vehicle normally causes injury to the occupants. If sufficient force is present, the pole is either bent or broken, depending upon the forces of impact and the strength of material used in the manufacture of the pole. Because of their circular or polygonal cross-section, such poles are extremely strong and the vehicle may endure extreme damage as a result of this strength. Such damage often results in injury and/or death to the occupants of vehicles striking such poles.
Many attempts have previously been made to provide breakaway couplings or connectors for the bottom ends of pole structures designed so that the pole structures will break away from their supporting foundations if the pole structures are impacted by a vehicle. Upon impact by a vehicle, these couplings act as weak points, reducing the amount of impact energy the pole structure can absorb, which leads to bending or breaking at a lower overall impact energy. This design results in a pole structure that will bend or break more easily upon vehicular impact, rather than remaining upright and causing further vehicular damage and passenger injury/death. The forward motion of the vehicle also forces the base of the pole structure forward, resulting in a flipping motion that causes the pole structure to gain sufficient height to pass over the roof of the vehicle. In addition to permitting the pole structure to break away from its supporting foundation, the breakaway couplings must also be strong enough to withstand the weight of the pole structure and wind loading. Furthermore, it is advantageous to have breakaway couplings that fail at a specific height sufficient to avoid tearing in the undercarriage of a vehicle. Such tearing can lead to a ruptured fuel tank, which in turn can cause fire and explosion.
There are several types of breakaway couplings found in the prior art and used commercially. For example, U.S. Pat. No. 6,910,826 discloses a breakaway coupling designed with longitudinal slits to enable easy rupture of the coupling when impacted. Although the coupling is designed to be very frangible, the longitudinal grooves enable the coupling to failure along the vertical axis, as opposed to circumferential grooves that would allow horizontal failure. Vertical failure could create problems if the coupling fails incompletely and sections of the coupling are left attached to the supporting foundation bolts. These coupling sections could extend the entire length of the coupling, which would exceed a safe height and could cause tearing of the undercarriage of a vehicle. Furthermore, the longitudinal grooves are prone to develop into cracks that lead to moisture intrusion, intergranular corrosion, galvanic corrosion, and crevice corrosion. This deterioration can cause the coupling itself to easily fracture when subjected to low wind speeds or from incidental forces resulting from very minor vehicle impact. In some cases, the internal corrosion alone can cause failure in the coupling since the corrosion products cause the coupling to split apart. This coupling design has great potential to cause many pole structures to fall down without an impact force, which could cause injuries and traffic stoppage due to blocked highways and expressways.
U.S. Pat. No. 4,638,608 discloses a coupling assembly with numerous interlocking body elements held together by two tension straps located at either end of the coupling. While the ends of the coupling have reduced diameters, the coupling lacks a horizontally disposed notch of any kind. In operation, the coupling is designed to fail at the tension straps. If only the top tension strap fails, the remaining strap could hold the sections of the coupling together, which could cause the coupling to exceed a safe height and cause damage to the underside of a vehicle. Furthermore, the coupling is also susceptible to several types of corrosion at the straps and in between the multiple body elements. Corrosion can cause the entire coupler assembly to fail due to low wind speeds or minor impact force. A corroded coupling can also collapse by bursting apart, causing early failure, which limits this coupling's usefulness in terms of reliability. Finally, this coupling design is dependent on multiple small pieces, which makes the coupling inherently more expensive to manufacture, due to machining costs for each individual piece.
U.S. Pat. Nos. 4,007,564 and 4,052,826 together disclose several types of breakaway coupling assemblies, the most relevant of which is a coupling constructed with surface grooves that extend part of the length of the coupling. Again, this design relies on failure along the vertical axis of the coupling, rather than horizontal failure, as in the case of a horizontal groove. If the vertical grooves cause the coupling to incompletely break apart from the supporting foundation bolts, a long coupling shard could remain to cause damage to the undercarriage of the vehicle. Additionally, the presence of multiple grooves can lead to corrosion and premature failure of the coupling, either from normally negligible wind and impact forces, or from internal failure due to corrosion itself. Since the grooves of the coupling are intricately positioned and sized, this design would increase the machining costs, which would make the coupling more expensive to produce. Furthermore, if the grooves of the coupling are not positioned correctly in relation to the anchor bolts, there is an increased risk of performance issues due to improper installation. The coupling may not fail as desired, leading to an increased amount of effort and associated labor cost required to install the couplings correctly due to the complicated design.
U.S. Pat. No. 3,630,474 discloses a breakaway coupling with a circumferential groove designed to enable the coupling to fail along a horizontal axis upon impact. This design makes the coupling frangible, but only upon a large impact force due to the nature of the internal bore of the coupling. The bore does not extend the entire length of the coupling, leaving a solid metal section of the coupling. This solid section is aligned with the circumferential groove, and upon impact the coupling is designed to fail at this section. Even with the position of the groove, however, a stronger force would be required to break the coupling than if the bore extended the entire coupling length, which could lead to increased vehicle damage and passenger injury. Precise and predictable failure may be unattainable with this design of coupling due to the solid metal core. The coupling may fail incompletely, or leave ragged fragments of solid metal, which severely limits the utility of the coupling. The solid section in the bore can lead to increased tap breakage, which can produce quality control issues. Additionally, tap breakage can lead to machine down time, coupling destruction, and tap replacement, which would increase the overall cost of manufacturing.
U.S. Pat. No. 3,837,752 discloses a breakaway coupling designed with groups of at least one circumferential groove located near the upper and lower ends of the coupling. If only the upper groove(s) fail, the length of coupling left on the supporting foundation anchor bolts would exceed the 4 inch AASHTO standard safe height and could cause tearing and other potential damage to a vehicle or rupture its fuel tank. Additionally, in this design, the anchor bolts are inserted past the circumferential grooves. This specification strengthens the coupling and makes it more resistant to impact, which increases the force required to cause the couplings to fail. In turn, this force is partially absorbed by the vehicle, causing additional damage and potential passenger injury.
Furthermore, typical materials of construction employed in the field can cause additional performance problems for the breakaway couplings described above. For example, breakaway couplings constructed of various steel or aluminum alloys are extremely susceptible to the types of corrosion discussed above, especially when aluminum alloys are paired with carbon steel anchor bolts. These materials are commonly used in the field. Additionally, the impact toughness of steel, plastic and certain aluminum alloys often vary widely with temperature. Over the range of −20 degrees Fahrenheit to 120 degrees Fahrenheit, a temperature range that applies to most of the United States, the impact toughness for some steels can vary as much as 80 ft-lbs. Couplings made of these materials will fail at widely varying impact forces, depending on the outside temperature. Couplings made of very strong materials can remain intact during a high impact collision, causing vehicular damage and extreme passenger injury. Moderate winds could cause the couplings to fail due to corrosion or cyclic fatigue. These typical materials, depending on which alloy is selected, can be difficult to machine, causing excess wear and tear on production equipment. Recycling these typical materials is often not easily feasible, which in combination with the cost of replacing worn machining parts, leads to a relatively higher cost of production.
While the devices of the prior art patents may represent different utilities in the art, there are still many deficiencies. For example, there remains a need for breakaway couplings that are durable enough to withstand the normal conditions of use. Breakaway couplings should be designed to withstand normal wind shear and incidental vehicle impacts. In these situations, it is ideal for the breakaway couplings to remain intact instead of failing prematurely and causing unnecessary vehicle or property damage.
There also remains a need for breakaway couplings that are corrosion resistant. Couplings should be able to withstand exposure to environmental conditions without degrading past the point of being useful. Since these couplings are usually installed outside, they should be manufactured of a material that can withstand rain, salt spray and other wet conditions without corroding and failing prematurely.
There remains a need for breakaway couplings that are also easy to manufacture. These couplings should be of the simplest design possible and made of a material that does not inflict excessive amounts of wear and tear on the machines used to produce them. The material of construction should not only be easily machinable, but precisely machinable as well. Preferably, the material used should also be recyclable to reduce the amount of waste metal and to lower the overall cost of production.
Additionally, there remains a need in the art for breakaway couplings that are easy to install. Breakaway couplings should be designed as simply as possible, without multiple parts to unnecessarily complicate the installation process. Simply constructed breakaway couplings require less time to install and contain less inherent risk of improper adjustment, which helps lessen the risk of performance issues due to improper installation.
Finally, there remains a need in the art for breakaway couplings that are reliable in terms of predictable frangibility. Breakaway couplings should be designed to fail in a predictable way that leaves a short length of coupling on the supporting foundation. This length of coupling should be short enough not to cause additional unnecessary vehicle damage, specifically scraping of the undercarriage and possible fuel tank rupture, which can result in fire and explosion. Breakaway couplings should also fail in a way that leaves a precise and predictable shape of coupling on the foundation, not a ragged, irregular coupling fragment that may cause additional damage. Additionally, these couplings should not be so rigid as to remain intact after a specified impact force. In this situation, the couplings should fail rather than remaining intact and causing additional vehicle damage and passenger injury.